Measuring and/or monitoring skin blood flow is important in various medical applications, for example, in obtaining information about skin function, in treating burnt skin, skin ulcers, applying skin grafts, and in evaluation of peripheral hemodynamics.
It is known in the art to measure skin blood flow non-invasively by locally heating an area of skin and measuring a temperature difference along the surface of the skin. The heated area is cooled by a variety of heat transfer mechanisms, inter alia, conduction through and along the skin and convection by the skin blood flow. The known, assumed or measured variables include the input heating or cooling power, the temperature difference and a heat transfer factor which takes into account the various aforementioned heat transfer mechanisms. These known, assumed or measured variables are input into well known heat transfer equations to calculate the unknown blood flow.
It is known in the art to perform the aforementioned procedure with a sensor shaped like a disk. The disk includes two main elements: a centrally located heating and measuring element and a peripherally located reference measuring element. The temperature difference is measured and monitored between the central heating element and the peripheral reference element.
In one type of disk sensor, the heat power input is held constant and the temperature difference is measured. Such an arrangement is described, for example, by A. V. J. Challoner, "Accurate measurement of skin blood flow by a thermal conductance method", Medical and Biological Engineering, 1975, 13:196-201, and by S. Thalayasingam and D. T. Delpy, "Thermal clearance blood flow sensor--sensitivity, linearity and flow depth discrimination", Medical and Biological Engineering & Computing, 1989, 27:394-398.
In another type of disk sensor, the temperature difference is held constant and the heat power input is measured. Such an arrangement is described, for example, by A. Dittmar, "Skin Thermal Conductivity", in J. C. Leveque, ed., Cutaneous Investigation In Health And Disease, New York, Marcel Dekker, 1989, 323-335, and by P. M. Greenhalgh, J. R. Jones, and J. S. Yudkin, "The 57 mm thermal clearance probe: a non-invasive tool for measuring subcutaneous blood flow", Clinical Science, 1989, 77:121-127.
Other publications which discuss skin blood flow measurement are Arnost Fronek, "Noninvasive evaluation of the cutaneous circulation", Chapter 27, pp. 269-279, in Vascular Diagnosis, ed. by E. F. Bernstein, published by Mosby; Pierre G. Agache and Anne-Sophie Dupond, "Recent Advances in Non-invasive Assessment of Human Skin Blood Flow", Acta Derm Venereol, 1994, Suppl. 185:47-51; M. Nitzan et al., "Theoretical Analysis of the Transient Thermal Clearance Method for Regional Blood Flow Measurement", Medical and Biological Engineering & Computing, 1986, 24:597-601; M. Nitzan et al., "Faster Procedure for Deriving Regional Blood Flow by the Noninvasive Transient Thermal Clearance Method", Annals of Biomedical Eng., 1993, 21:259-262; G. Delhomme, W. H. Newman, B. Roussel, M. Jouvet, M. F. Bowman and A. Dittmar, "Thermal Diffusion Probe and Instrument System for Tissue Blood Flow Measurements: Validation in Phantoms and in Vivo Organs", IEEE Transactions on Biomedical Engineering, Vol. 41, No. 7, July 1994; and F. Arnaud, G. Delhomme, A. Dittmar, P. Girard, L. Netchiporouk, C. Martlet, R. Cespuglio and W. H. Newman, "A Microthermal Diffusion Sensor for Non-invasive Skin Characterization", Sensors and Actuators A, 41-42 (1994). In the latter reference, a transient thermal method was used.
French Patent 85-15-932 to CNRS (National Center of Scientific Research), Lyon, France describes a constant temperature difference sensor and was used in the above-mentioned work of Dittmar. This disk sensor has several drawbacks. First, the principle of maintaining a constant temperature difference and measuring heat power input is correctly applied when the reference temperature is constant, such as the temperature of the blood flowing underneath the skin where the sensor is attached. However, in the disk sensor the reference temperature is at the periphery of the disk. The peripheral temperature is not constant, but rather increases during measurement due to heat transferred from the central heater to the periphery of the disk. Second, this sensor has a plurality of thermocouples included between the central and peripheral portions of the disk which adversely affect the accuracy of the measurement by shunting heat away from the skin. Third, the sensor includes more than 20 elements from 6 different materials, greatly increasing manufacturing costs.
French Patent Document 9011773 to Dittmar et al. describes a sensor similar to that described in the above-mentioned French Patent 85-15-932. While the Dittmar sensor has been improved with advanced materials and technology, it retains basically the same drawbacks as in the previous sensor.
Japanese Patent No. 6-217952 describes a skin blood flow sensor which has an inner disk for temperature measurement and an outer ring and heating or cooling elements. This sensor, which has a structure that is generally similar to those of French Patent Documents 85-15-932 and 9011773, and thus also disadvantages similar thereto.